1. Field of the Invention
This invention relates to a process for purifying a crude olefinically unsaturated nitrile such as acrylonitrile and methacrylonitrile containing hydrogen cyanide and water by removal of the hydrogen cyanide. Also, it relates to a condenser useful for the same process.
2. Description of the Prior Art
In the conventional process, the reaction gas obtained in the ammoxidation of propylene or isobutylene is fed to a quenching column where unreacted ammonia is removed. Then the reaction gas quenched is counter-currently contacted with absorbing water in an absorption column, and a reaction exhaust gas containing unreacted propylene or unreacted isobutylene, carbon monoxide, carbon dioxide, nitrogen etc. is dicharged from the top of the absorption column while an aqueous solution mainly containing acrylonitrile or methacrylonitrile, acetonitrile and hydrogen cyanide is withdrawn from the bottom of the absorption column. The aqueous solution thus obtained is fed to an extractive distillation column, to the upper part of which water is introduced as an extractant. The distillate from the top of the extractive distillation column is condensed and an acrylonitrile or methacrylonitrile solution containing hydrogen cyanide and water is recovered. Then the acrylonitrile or methacrylonitrile solution after removal of non-condensing gas is fed to a rectifying column and from the top of the rectifying column is recovered hydrogen cyanide while from the bottom of the rectifying column is recovered acrylonitrile or methacrylonitrile and water.
The concentration of hydrogen cyanide in the vapor obtained from the top of the rectifying column is high and the cooling temperature of a condenser must be maintained lower than the boiling point of hydrogen cyanide. For this reason, cooling water from a cooling tower which is usually employed is not suitable as the cooling medium, and accordingly, water refrigerated with a cooling medium such as ammonia or freon has been used, resulting in an increase in power consumption.
According to another process for conducting the rectification at a high efficiency as described in U.S. Pat. No. 3,507,755, a crude acrylonitrile is fed into a distillation column at a point just above the middle of the column and the liquid within the column is withdrawn just below the point of introduction of the crude acrylonitrile, cooled and separated into two phases, i.e. an upper organic phase rich in acrylonitrile and a lower aqueous phase rich in water. The lower aqueous phase is withdrawn while the upper organic phase is returned to the column at a point just below the point of withdrawal of the liquid. In this process, by-produced impurities such as acetone, acrolein and acrolein cyanohydrin which are contained in the lower aqueous phase can be removed together with water, and as a result, from the bottom of the column is obtained an acrylonitrile solution having a much more improved purity than that obtained by the conventionally employed process while from the top of the column is obtained hydrogen cyanide having a higher purity. Further, a comparative reduction in consumption of steam for heating the column can be achieved. However, in this rectifying process, the concentration of hydrogen cyanide in the vapor from the top of the column reaches at least about 96% by weight and the temperature of the top of the column becomes about 30.degree. C. due to the boiling point of hydrogen cyanide of 25.7.degree. C. and accordingly, refrigerated water having a low temperature is also required as the cooling medium for the condenser in an atmospheric pressure operation, resulting in a marked increase in cost of power and equipment for the refrigerator for cooling. In order to condense the vapor having such a temperature with ordinary cooling water, an extremely large area of heat transmission for cooling is required due to the small difference of temperature between the vapor and the cooling water. As a result, the industrial feasibility of this process is small in spite of its advantages. On the other hand, it may be considered that operation of the column under a higher pressure renders cooling for condensation easier due to increased temperatures of the top of the column, but the temperature of the bottom of the column is simultaneously increased. As a result, polymerization of acrylonitrile in the liquid of the column is disadvantageously increased for practical purposes.
In general, when a vapor containing non-condensing gas is condensed, its condensation is conducted while withdrawing the non-condensing gas from an outlet of the condensation system provided at a suitable point. Such a point is usually the top of a condenser. However, when the condensation is conducted by a condenser having an outlet at its top, at least two series of control systems, one for the level of the condensate liquid and the other for the amount of the vapor fed to the condenser or the amount of the condensate liquid withdrawn, are required for controlling the amount of condensation or the condensation of vapor at a constant flow rate. Several methods are employed for this purpose. For example, the amount of the vapor fed is controlled at a constant flow rate and the amount of condensation is controlled in such a manner that the level of the condensate liquid is kept constant. Or the flow amount of cooling water in the condenser is controlled in order to maintain the amount of vapor constant, and the flow amount of the condensate liquid is controlled in such a manner that the level of the condensate liquid is kept constant. In the above described methods, any regard is not given to the control of withdrawing non-condensing gas.